Despite increased HDL-C, LCAT transgenic (L-Tg) mice have increased aortic atherosclerosis (athero) due to accumulation of dysfunctional HDL with impaired reverse cholesterol transport. L-Tg HDL is CE-rich with decreased apoA-I/CE ratio. To evaluate if overexpression of apoA-I can normalize L-Tg HDL function and decreased athero in LCAT-Tg mice we crossbred hL-Tg with hApoA-I-Tg mice. Compared to L-Tg mice, L-Tg x apoA-I Tg mice(L x AI-Tg,n=14) had similar LCAT activity (1054+/-41 nmol/ml/h) and 6-fold increased in total apoA-I (660+/-44 mg/dl). On a regular chow diet (RCD) lipid values (mg/dl) for L x A-I -Tg were TC=1031+/-48,CE=813+/-36, HDL-C=930?100 and apoA-I=660+/-44 which were significantly increased (p<0.03) compared to apoA-I Tg (n=13) by 82%(TC), 84%(CE), 84%(HDL-C), and 57%(apoA-I) and increased (p<0.03) compared to L-Tg (n=49) by 76%(TC), 78%(CE),79%(HDL-C), and 83%(apoA-I). After 16 weeks on a high fat-high cholesterol diet(HFD)the lipids in the L x A-I Tg(1097+/-100) were greater than apoA-I Tg(430+/-48,p<0.001)and L-Tg(548+/-73,p<0.002). FPLC analysis both on a RCD and HFD demonstrated marked accumulation of larger, CE-rich HDL in L x A-I Tg compared to L-Tg and apoA-I Tg. The mean aortic lesion size (um2x 103) was significantly decreased p<0.001) in L x A-I Tg (10.7+/-1.3;n=14) compared to L-Tg (35.7+/-2;n=49) to a level = to that of CO (11.5+/-1;n=28). Thus, ApoA-I and LCAT act synergistically to profoundly increased TC 4-fold in L-Tg mice to levels (1000mg/dl) seen to date only in apoE-def mice. Dramatic accumulation of larger, CE-rich HDL accounts for the increased TC on both a RCD and HFD. Compared to L-Tg, athero in L x A-I Tg is decreased by 70 % to a level similar to CO. Overexpression of human apoA-I alters hL-Tg HDL composition and function which may prevent the formation of dysfunctional HDL, enhance reverse cholesterol transport and thus, reduce atherosclerosis in LCAT transgenic mice.